JN

JN

JN

4 000674 00066

68

000

81

68

00

082

UTM zone 6 - NAD83, topo basemap “Talkeetna Mountains (A-2 SE) 1:25,000

1517

10

15

2515

A

A’

Bubb Creek fault

Nelchina Limestone(Lower Cretaceous)cliff-forming clastic limestone

“Bedlamite” (Lower Cretaceous)*calcareous mudstone

“Seabee” (Lower Cretaceous)*recessive, yellow sandstone, coal seams

“Wittestone” (Lower Cretaceous)*recessive, siltstone

“Garstone” (Lower Cretaceous)*moderately recessive brown sandstone

500 m

Naknek Formation (UpperJurassic) sandstone,mudstone, conglomerate

JN

*The names are informal.

1600

1500

1400

elevation [m]A

W EA’

deformed belemnite locality

Bubb Creek fault

5 m

Bedlamite

Bedlamite

Nelchina Limestone

Bedla

mite

Nelchina Limestone

Nelchina Limestone

GarstoneGarstone

Garstone

Wittestone

W E

NW SENelchina Limestone

Contractionally deformed belemnites as indicators for thrust faulting in mechanically weak sedimentary layersAn example from the Lower Cretaceous of the Talkeetna Mts., Central AlaskaJochen E. MezgerDepartment of Geosciences, University of Alaska FairbanksFairbanks, AK, 99775, USA - jemezger@alaska.edu

50 km

Limestone Gap

Castle Mountain

Fault

62°

61°

147° W

fault zone

Ranges

Border

lt zou na ef anitniTenoz tluaF ilaneD

Border Rangesfault zone

CT

WC

T

References:Badoux, H. (1963). Les bélemnites tronçonnées de Leytron (Valais). Bulletin de la Société vaudoise des Sciences naturelles 68, 233-239.Beach, A. (1979). The analyses of deformed belemnites. Journal of Structural Geology 1, 127-135.Hossain, K. H. (1979). Determination of strain from stretched belemnites. Tectonophysics 60, 279-288.Hüttner, R. (1969). Bunte Trümmermassen und Suevit. Geologica Bavarica 61, 142-200.Lloyd, G. E. & Ferguson, C. C. (1989). Belemnites, strain analysis and regional tectonics: a critical appraisal.

Tectonophysics 168, 239-253.Passchier, C.W. & Trouw, R.A.J. (2005): Microtectonics. 2nd edition. Berlin: Springer.Ramsey, J. G. (1967). Folding and fracturing of rocks. New York: McGraw-Hill.Ramsay, J. G. & Huber, M. I. (1983). The techniques of modern structural geology. Volume 1: Strain Analysis. London:

Academic Press.Trop, J. M. (2008). Latest Cretaceous forearc basin development along an accretionary convergent margin: South-central

Alaska. Geological Society of America Bulletin 120, 207-224.Wilson, F. H., Hults, C. P., Mull, C. G. & Karl, S. M. (2015). Geologic Map of Alaska. U. S. Geological Survey Scientific

Investigations Map 3340, pamphlet 197 p., 2 sheets. Scale 1:1,584,000.

2 mm

1 mm

1 mm

1 mm

1 mm

1 mm

Und

efor

med

ros

trum

with

dar

k gr

owth

line

s pa

ralle

l to

long

axi

s ...

Cal

cite

cry

stal

s fo

rmed

in a

frac

ture

ope

ned

in a

ben

t bel

emni

te ros

trum

;

sam

e sp

ecim

en a

s in

the

left

phot

omic

rogr

aph

... and calcite crystals growing perpendicular to long axis

Calcite vein with blocky crystals in a bent belemnite showing syntaxialgrowth, i.e. crystals grew from the median line (see photograph on the left )

High angle (w

ith respect to rostrum long axis) fractures w

ith displacement;

few recrystallized calcite

Calcite vein w

ith blocky crystals indicate fast opening;

fine grained crystals in the center of the vein suggest second opening phase

Photomicrographs of undeformedand deformed belemnite rostra.

All images taken undercrossed polarized light.

median line

grow

thdi

rect

ion

smaller 2nd opening

phase calcite crystals

75°

60°

35°

43°

40°

Bent

Sheared (high angle)

Sheared (low angle)

Flattened

ReX

ReX

ReX

Slic

ken

lines

ReX

ReX

ReX

ReX

MUSEUMNORTHTHE

OF

CM

MUSEUMNORTHTHE

OF

CM

ReX

ReX

pressure solution

pressuresolution

Bent29.5% (36)

Flattened9% (11)

Sheared(low angle)

18% (22)

Sheared(high angle)43.5% (53)

Deformed belemnitesTotal N = 122

0°

90°

180°

270°

Shear angles relative torostrum long axis

(5° classes)

N = 82

rostr

um

lon

g a

xis

83°69°

38°

Regional geologic map showing the fossil locality and the proximity of the belemnite-bearing “Bedlamite” calcareous mudstone to reverse and thrust faults that cut across the limestone-marl-sandstone sequence. The dip of the reverse faults is not exactly known and only estimated from exposure-topography relations.

Eastward dipping sedimentary sequence of the resistant Nelchina Limestone, the more recessive Belamite calcareous mudstone, the Garstone sandstone and the Wittestone. The Bedlamite outcrop is the only locality with a decent exposure of the calcareous mudstone. In other places, strong susceptibility to weathering results in recessive erosion, overgrown slopes and concealed by sandstone debris from the overlying Garstone unit. Belemnites are washed out of the mudstone and accumulate in the gully formed at the contact with the underlying hard Nelchina Limestone. Note the westward directed reverse fault (red line with arrows pointing downdip) cutting across the Bedlamite-Garstone contact.

The study area (Limestone Gap) in the Talkeetna Mts. comprises Middle Jurassic-Oligocene sedimentary and volcanic rocks of the accreted Talkeetna arc, part of the Wrangellia composite terrane (WCT). The Border Ranges fault zone juxtaposes the Talkeetna arc against the accretionary prism of the Chugach terrane (CT). Geologic map modified after Wilson et al. (2015).

The calcareous mudstone of the Bedlamite is extremely susceptible to weathering, due to its high calcite content. However, after digging half a meter into the slope a much stronger less altered mudstone is revealed. It appears to be very brittle and fractured. It is not possible to expose continuous bedding layers.

Possible formation of belemnite fracture shears and associated tectonic model. The thin and relative weak calcareous mudstone is wedged between stronger massive layers (Nelchina Limestone and/or Garstone) and serves as glide horizon in duplex structures. Internal deformation within the mudstone accommodates some of the strain resulting from west-directed compressive stress.

Acknowledgments:Thanks to Nathan Graham (UAF) for

providing some of the deformed belemnites. Belemnite pictures were

taken at the Museum of the North, University of Alaska Fairbanks, courtesy

of Pat Druckenmiller.

Introduction

Stretched belemnites, more precise, the rod-shaped guards or rostra that form the internal skeleton of the rear third of a squid-like cephalopod, have been observed in Jurassic slate and limestone of the French and Swiss Alps. The rostrum is made up of radiating fibrous calcite crystals oriented with their long axes perpendicular to that of the rostrum (see photo above), resulting in low tensile strength. When subject to extension, the belemnites fracture and form boudins, i.e. they deform in a brittle fashion and the spaces between the fragments are filled by quartz/calcite fibers or matrix material. Elongation can easily be determined and used for strain analyses (Badoux 1963; Ramsay 1967; Hossain 1979; Beach 1979; Lloyd & Ferguson 1989). Many students are probably familiar with the practical strain measurement problems involving belemnites in volume 1 of Ramsay & Huber’s “Techniques of Modern Structural Geology”.

In contrast to stretched belemnites, there is very little to be found in the scientific literature about belemnites having experienced contractional deformation, with bent or sheared rostra. It is unclear if that is owed to an actual scarcity or reflects a bias, i.e. indifference of structural geologist towards fossils and invertebrate palaeontologists towards deformation.

The only reference I am aware of dates to Hüttner (1969) who described sheared belemnites from Jurassic shale within the Ries meteorite crater in Bavaria. Hüttner concluded that the deformation was the direct result of the meteorite impact in the Miocene! Recent finds in central Alaska which are presented here question this theory.

Limestone Gap in the Talkeetna Mountains

The rocks that underlie the Talkeetna Mountains in south-central Alaska are part of the large Wrangellia composite terrane (WCT), which accreted to North America in the late Mesozoic. The Talkeetna Arc comprises Jurassic oceanic arc sediments, Cretaceous-Oligocene forearc basin sediments and Eocene volcanic rocks (Trop 2008). Subsequent accretion of the Chugach terrane to the south and the development of major strike-slip faults in the Paleogene (Denali and Castle Mountain faults) are testimony to significant longlived tectonic activity. The immediate study area in the eastern Talkeetna Mountains is controlled by kilometer-scale open folds of the massive and resistant Nelchina Limestone, forming plateaus that cap the kilometer thick sequence of the Naknek Formation. Locally known as Limestone Gap, the ideal morphology and the proximity to the Glenn Highway are the reasons why this area is the location of the capstone mapping project of the Geology Field Camp of the University of Alaska Fairbanks.

The deformed belemnites of Limestone Gap

The calcareous “Bedlamite” mudstone is very recessive, its calc-rich matrix easily dissolved and the remaining clay

washed away. Left behind and concentrated on the present surface or the top of the underlying massive limestone are

numerous fragments of belemnite rostra and mollusk shells. The belemnites belong to the lower Cretaceous species

Cylindroteuthis. Fragments are up to 10 cm long with diameters up to 5 cm. The overwhelming majority of the

fragments are broken perpendicular to the rostra long axes or split parallel to it.

Thin section analyses

Thin sections were prepared from several sheared and bent belemnites. Undeformed parts of the rostrum

show dark thin lines parallel to the long axis, representing growth rings. The calcite crystals that

form the rostrum are up to 1 mm wide and several mm long, growing radial and perpendicular to the long

axis. Fractures are developed as sub-millimeter wide

micro fault zones with fragments of the rostrum rotated in the manner of fault blocks. Displaced growth

lines indicate displacement and sense of motion. Fractures with newly crystallized colorless calcite

contrast with the brown calcite of the skeleton. Their parallel and elongated prisms indicate growth

perpendicular to the fracture, characteristic of dilatation sites (mineral veins). Their general blocky

habitus suggests a fast opening syntaxial growth (Passchier & Trouw 2005). Smaller crystals around

the median line indicate a second opening phase.

Amongst short belemnite rods, the occasional odd-shaped belemnite fragment - folded, flattened or sheared

- stand out. The abundance of such deformed belemnite rostra is difficult to estimate, but several hours of

collecting at two localities yielded over 120 specimen. Representative rostra of the nicest specimen covering the

variety of deformation types are shown on the main photograph plate, along with interpretive sketches

outlining the deformation structures.

The majority of the samples (60%) are sheared, predominantly with shear planes at high angle to the

rostrum long axis. Others resemble shear fractures (< 40° to long axis) developed under the Coulomb fracture

criterion in experimental rock deformation experiments. A third of the rostra are bent, up to an angle of 75°. A

small number (9%) of belemnites are flattened. Some samples show two generations of shear

fractures, high angle shears being overprinted by low angle Coulomb shear fractures.

All specimen have in common that the deformed rostra are coherent, held together by recrystallized calcite, which

can be distinguished by its light grey to white color from the brown of the original rostrum. Fractures on the

shortening side of folded belemnites are indicative of pressure solution.

Overlying the Nelchina Limestone are belemnite-bearing calcareous mudstone (”Bedlamite”), sandstone (”Garstone”), siltstone (”Wittestone”) and sandstone with coal seams (”Seabee”). None of these units have been mapped individually. They are included in the Nelchina Limestone map unit of Trop (2008). The informal names used in this poster originate from the UAF Geology Field Camp.

Help!

Do you havedeformed belemnites,

know someone who does,know of any publications

or field localities?I would love to hear from you!

Contact me: jemezger@alaska.edu

Thank you!!

Strain markers?After death of a belemnite, its rostrum came to rest horizontally on the surface of the ocean floor, and assuming the absence of strong bottom currents, without alignment. After diagenesis, the rostrum’s orientation is parallel to bedding, but random in X and Y directions.

The presence of calcite veins - several generations in some samples - preserving the coherence of the deformed belemnite rostrum strongly suggests that the fossils experienced continuous deformation over a certain period.

Unfortunately, the outcrop conditions do not reveal any deformation fabrics within the Bedlamite mudstone. Also, the belemnites are not found in place at the Limestone Gap outcrops, not allowing reconstruction of in situ orientation of the rostra or strain analyses. Assuming random orientation within the bedding plane, the high and low angle shear fractures can be attributed to the initial different orientation of the rostra long axes. Both high and low angle shears can be explained by one stress direction (σ1). Overprinting shear fractures could hint at changes in the main compressive stress or rotation of the fossil. Some rostra are crushed, indicating that flattening (related to diagenesis or vertical tectonic stress?) also affected the fossils. Some high angle shear fractures might be the result of flattening. Clearly, there is still much to investigate.

The Bedlamite calcareous mudstone layer is thinner (10-15 m) and weaker than the underlying Nelchina Limestone (30-40 m) or the overlying Garstone sandstone (>40 m). The tectonic setting in Limestone Gap is very complicated, with several E-W striking strike-slip faults and N-S striking reverse/thrust faults with west-directed shortening. Several reverse faults cut the Nelchina-Bedlamite-Garstone unit and form duplex structures. At the fossil location, a reverse fault places Nelchina Limestone onto the calcareous mudstone, effectively wedging it between the strong limestone. It is very likely that the relative weak mudstone accommodates much of the strain by means of internal deformation.

The mudstone, though apparently soft at the weathering surface, is competent to enough to prevent complete strain partitioning between matrix and belemnite rostrum. In other words, the belemnites are sufficiently attached to the matrix to respond to the strain forced upon the mudstone. Thus, the deformed belemnites can serve as strain indicators.

Tectonic indicators? What useful information can be extracted from deformed belemnites that we don’t already know?Conditions in other areas might not be favourable enough to preserve regional structural features as in south central Alaska. Belemnites are very resistant to weathering. In such cases, sheared and bent belemnite rostra might be the only indicators of compressive tectonic stress. In order to validate this conclusion, more contractionally deformed belemnites need to be discovered. However, in Alaska we can exclude an impact of an extraterrestrial body as the origin of sheared belemnites!

W ENelchina Limestone

planar view

X-section view

σ1

σ1